This invention relates to a method and apparatus for synchronizing the speeds of rotation of gyroscope flywheels. More particularly, the invention relates to improved methods and circuits for use in phase locking one or more gyroscope wheels at start-up.
An individual gyroscope instrument usually consists of a flywheel (momentum ring) and a motor for driving and maintaining the angular velocity of the wheel at a desired, precise level. Such an instrument often exhibits unwanted drift phenomena as a result of interaction between the flywheel and its motor. Thus, mechanical vibration due to inperfections in the ball bearings, raceways, and retainers results in synchronous vibration which acts upon various finite sensitivites of the flywheel itself and causes unwanted drift behavior in the instrument.
These mechanical effects can be minimized in a gyroscope by appropriate calibration of the instrument, accompanied by appropriate compensation. However, the economics of production often limit the degree by which the sensitivities can be reduced. Therefore, other techniques, such as phase locking, are employed, which are aimed at providing operational stability of the gyroscope by creating a repeatable and stable set of vibration profiles. In this way the inherent residual sensitivity of the gyroscope to synchronous vibration is maintained constant.
Phase locking provides for synchronization of the gyroscope at each start-up so that a sustained, repeatable, operating environment is provided. In general, this technique consists of determining the position, relative to a fixed point, of an arbitrary reference point on the gyroscope flywheel, and of aligning this point to an external reference signal wave form. This is done by means of a flywheel reference signal which is indicative of the angular position of the wheel about the spin axis of the gyroscope.
When a synchronous hysteresis motor is used to drive the gyro, electromagnetic coupling due to "side-pull" causes transverse and axial vibrations, which result from inherent imperfections in the rotor and motor-winding assembly. These vibrations are, by design, inherently synchronous and produce a drift of the gyroscope angle which can be shown to be directly related to the mechanical angle at which the motor attained synchronous speed. Since, for a normal synchronous hysteresis motor, this angle is arbitrary, it is desirable to provide an angle of synchronism which is repeatable each time the instrument is turned on so that repeatable drift performance is assured.
Techniques which are currently in use for reducing motor side pull also employ phase locking. Here, "interruption approaches" are used, in which the speed of rotation and the synchronization angle of a gyroscope are modulated until a desired angle between the motor voltage and a reference signal emanating from the flywheel is achieved. These techniques are complex to implement and often unreliable, since they depend upon motor dynamics, that is, acceleration or deceleration of the flywheel, to achieve the results, and are subject to variation from instrument to instrument.
In constructing platforms for use in inertial navigation packages, it is usual to employ a number of gyroscopes on a single platform. When so used, the gyroscopes may either be strapped down, that is, used without gimbal mounting, or they may be mounted in gimbals carried on the platform. The mounting of a number of gyroscopes on the same platform couples the gyroscopes together mechanically and mechanical cross-talk occurs between them due to the interaction of the synchronous vectors arising in each gyroscope. Further undesirable drift behavior is thus induced.